Reflecting surface and microwave absorptive layer



' ate REFLECTIN G SURFACE AND IMICROWAVE ABSORPTIVE LAYER Otto Halpern, Los Angeles, 'Calif., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Feb. '1, 1946, Ser. No. 644,975

. of affixing a high loss dielectric layer either directly to the surface to be protected or to a metallic sheet which Way, in turn, be so applied. Such protective coatings generally have a thickness equal to an odd multiple of a quarter-wave length of the incident radiation for which the layer is designed to be resonant.

In my copending application entitled Method and Means for Minimizing Reflection of High-Frequency Radio Waves, Serial No. 581,179, filed March 5, 1945, now Patent No.- 2,923,934, there is disclosed a protective layer preferably consisting of myriads of electrically conductive flake-like particles dispersed quasi-insulated in a nonconductive binder. Electrically conductive particles thus arranged will impart to the whole medium a dielectric constant which will be greatly in excess of that of the binder alone. In order to obtain a high dielectric constant, it has been found to be desirable to employ the flake-like metallic particles in dense concentration so oriented in the binder that the plane of the large dimension of the flakes is parallel to the aggregate. However, considerable difliculty has been experienced in securing a layer in which the plane of the large dimension of the flakes is oriented parallel to the plane of the aggregate.

An object of this invention is to provide a radio microwave absorbent layer consisting of myriads of flake-like electrically conductive particles dispersed quasi-insulated in a binder having means deposited on the surface of such flakes to assist the flakes to assume a desirable orientation in the binder.

Another object of this invention is to provide a layer for minimizing reflection of incident radio microwave radiation consisting of myriads of flake-like electrically conductive particles dispersed quasi-insulated in a nonconductive binder in which the particles are provided with a lubricant coating to assist the flakes to assume a desirable orientation in the binder.

A further object of this invention is to provide a radio microwave absorbent layer consisting of myriads of flakelike metallic particles dispersed quasi-insulated in a binder in which the flakes are provided with a lubricant coating of from two to three percent by weight of the particles for reducing the surface tension thereof.

Other objects and advantages of this invention will be apparent from the following description and claims.

In the construction of protective layers for absorbing incident radio microwave radiation, metallic threads, powders, or flake-like particles, or ferromagnetic counterparts of the same may be used. However, it has been found advantageous to employ either metallic flake-like particles or ferromagnetic flake-like particles in practice. Conductors such as aluminum, copper, iron, steel, Permal- 10y, and graphite flakes and mixtures thereof may, for example, be used. Such flakes average in thickness be tween 3 10- and 2X10 centimeters with the long dimensions as high as seventy (70) times the average thickness. The flakes may be dispersed and thoroughly mixed in a variety of binder materials such as Waxes, rosins, polystyrene, commercially available Vistanex, or synthetic rubbers. Practically useful concentrations of the metal content vary between twenty .(20) and eighty percent by weight of the mixture.

While it has heretofore been stated that either electrically conductive flakes or ferromagnetic flakes may be dispersed throughout the binder, it is to be noted that the thickness of the two layers may be different. In the case where ordinary metallic flakes are employed, the thickness of the layer must be equal to an odd multiple of a quarter Wave length, measured inside the layer, of the incident radiation for which the layer is designed to be resonant. In the case Where ferromagnetic flakes are employed, the thickness of the layer need not in theory be equal to an odd multiple of a quarter wave length of the incident radiation, although it may be so constructed.

In order to form a radiation absorbent layer of the class described the composition of flakes and binder may be applied by mixing the flakes in the binder and working the surface of the mixture with a flat tool at room temperature or slightly higher. As heretofore stated, it is highly desirable that the flakes assume the preferred orientation in the binder, that is, with the plane of the large dimension of the flakes parallel to the plane of the aggregate. Working the coating by a smoothing or stroking motion aids in obtaining a more uniform leafing of the flakes with thin surfaces generally parallel to the plane of the layer. Heretofore it has been diflicult to obtain coatings in which the flake material assumes the desired orientation in the binder.

It has been discovered that if a small amount of lubricant be added to the flake particles either in the process of manufacture thereof or before mixing the flakes with the binder, the flakes much more readily orient themselves within the binder with the plane of the large dimension of the flakes parallel to the plane of the aggregate. The reason for this is that the thin film of lubricant on the surface of each flake aifects the surface tension thereof, lowering it sufficiently that the flakes much more readily tend to lie flat and leaf together more uniformly when the surface of the coating is subjected to a Working motion. Various types of lubricants such as greases and oils, either animal or vegetable, may be employed. In has been discovered, however, that the flakes most readily tend to lie flat and leaf together uniformly when a lubricant content of from two (2) to three (3) percent by weight of the metallic flakes is used.

While a particular embodiment of this invention has been disclosed and described it is to be understood that various changes and modifications may be made therein without departing from the spirit and scope thereof as set forth in the appended claims.

What is claimed is:

1. In combination with a normally reflecting surface, a layer for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of finelydivided electrically conductive particles dispersed quasiinsulated in an organic binder, said particles having a grease-type lubricant coating for reducing the surface tension thereof, the thickness of said layer being equal to an odd multiple of one-quarter wave length of the incident radiation, measured inside the layer.

2. In combination with a normally reflecting surface, a

layer for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of finelydivided electrically conductive particles dispersed quasiin sulated in an organic binder, said particles being provided with a grease type lubricant coating comprising'two to three-percent by weight of the particles for reducing the surface tension thereof, the thickness of said layer being equal to an odd multiple of one-quarter wave length of the incident radiation, measured inside the layer.

3Q In combination with a normally reflecting surface, a layer for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of finelydivided ferromagnetic particles dispersed quasi-insulated in an organic binder, said particles having a grease-type lubricant coating disposed upon the surfaces thereof for reducing the surface tension of said surfaces.

4. In combination with a normally reflecting surface, a layer for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of finely- 'divided ferromagnetic particles dispersed quasi-insulated in an organic binder, said particles being provided with a grease-type lubricantcoating comprising two to three percent by weight of the particles for reducing the surface tension thereof. a

5. In combination with a normally reflecting surface, a layer for minimizing the reflection of radio microwave radiation incident thereon comprising-myriads of-metallic flake-like particles dispersed quasi-insulated in an organic nonconducting binder, said flakes having a grease-type lubricant coating for reducing the surface tension thereof, the thickness of said layer being equal to an odd multiple of a quarter wavelength of the incident radiation, measured inside the layer. a

6. In combination with a normally reflecting surface, a layer for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of flakelike metallic particles dispersed quasi-insulated in an organic nonconducting binder, said flakes having a greasetype lubricant coating comprising two to three percent by Weight of the flakes for reducing the surface tension thereof, the 'trickness of said layer being equal to an odd multiple of a quarter wave length of the incident radiation, measured inside the layer. 7 V

7. In combination with a normally reflecting surface, a layer for minimizing the reflection. of radio microwave radiation incident .thereon comprising :myriads of flakelike ferromagnetic particles dispersed quasi-insulated in a nonconducting organic binder, said flakes'having a grease-type lubricant coating comprising two to three percent by weight of the flakes disposed upon the surfaces thereof for reducing the surface tension of said surfaces.

8. In combination with a normally reflecting surface, a layer of predetermined thickness for minimizing the reflection of radio microwave radiation incident thereon comprising myriads of electrically conductive particles coated with a grease-type lubricant and dispersed homogeneously throughout said layer.

References Cited in the file of this patent UNITED STATES PATENTS Faust Sept. 29, 1942 

1. IN COMBINATION WITH A NORMALLY REFLECTING SURFACE, A LAYER FOR MINIMIZING THE REFLECTION OF RADIO MICROWAVE RADIATION INCIDENT THEREON COMPRISING MYRIADS OF FINELYDIVIDED ELECTRICALLY CONDUCTIVE PARTICLES DISPERSED QUASIINSULATED IN AN ORGANIC BINDER, SAID PARTICLES HAVING A GREASE-TYPE LUBRICANT COATING FOR REDUCING THE SURFACE TENSION THEREOF, THE THICKNESS OF SAID LAYER BEING EQUAL TO AN ODD MULTIPLE OF ONE-QUARTER WAVE LENGTH OF THE INCIDENT RADIATION, MEASURED INSIDE THE LAYER. 